The present invention relates, in general, to a process for fabricating optical packages, and more particularly to a process for aligning an optical fiber with a photodetector surface in a housing during fabrication of the package.
In optical fiber systems, where data may be transmitted by optical energy such as laser light which travels through the fiber, the fiber typically terminates in a photonics device such as a receiver, transponder, transceiver, or the like, where optical signals are received and converted to corresponding electrical signals. Typical photonics packages include a radio frequency (RF) chip mounted on one platform, or carrier block, and a photodetector mounted on another platform, or carrier block, using standard die attachment techniques in each case. The two carrier blocks are interconnected at right angles to each other, for example, with the photodetector block including a thick film connection line for interconnecting the photodetector and the RF chip. Alternatively, the photodetector and the RF chip can be interconnected by a ribbon or wire bond. The optical fiber output end then is aligned with the photodetector to permit the transfer of optical energy from the fiber to the photodetector.
It has been found that in such optical packages, RF interconnection losses between the photodetector and the RF circuitry are unacceptably high, due in part to signal losses in the finite lengths of the bonded ribbon connectors used to electrically connect the photodetector to the RF circuitry. In addition, it has been found that in these prior photonics packages, an accurate alignment of the optical fiber and the photodetector is difficult, and this increases the potential for signal losses.
More particularly, an accurate alignment of an optical fiber with its target photodetector is necessary to maximize the illumination of the photodetector by optical energy from the output of the optical fiber. Typically, such alignment has been accomplished by placing a metallic shield on the optical fiber, clamping the shielded fiber to the surface of a chip carrier, or to a housing wall on which the carrier is mounted, and then positioning the fiber, using pneumatic tweezers, to direct the optical energy output from the fiber onto the photodetector. A major problem with this method is that after the fiber has been optimally positioned by the tweezers, the clamp must be welded to the carrier or housing wall to secure the fiber in place. It has been found that welding the clamp can cause movement of the fiber, however, resulting in optical misalignment of the photodetector and the fiber. When this occurs, it is difficult to realign the fiber with the photodetector, and a permanent loss of signal through the optical package can result.
Accordingly, there is a need for an improved photonics package, in which a photodetector and RF circuitry connected thereto are located on the same housing surface, as well as a need for an improved technique for fabricating the photonics package which permits better optical alignment of the photodetector and an input optical fiber.
In accordance with a preferred form of the invention, an optical package is fabricated by positioning an optical fiber in the wall of a housing, aligning it with a photodetector mounted in the housing, and adjusting the position of the fiber as it is secured to the housing in such a way as to ensure accurate alignment and the resulting maximum illumination of the photodetector by light energy from the fiber. The process of the invention allows the use of a monolithic integrated circuit chip that contains both a photodetector and the RF circuitry used with the photodetector, since the alignment technique of the invention does not require mounting of the optical fiber on the RF chip carrier. Further, the method of the invention allows the fiber to be aligned accurately with the small photosensitive surface of the photodetector chip, and to maintain its alignment while the fiber is secured to the housing. The method not only ensures maximum illumination, but by using an integrated circuit it minimizes RF losses between the photodetector and the RF circuitry.
In greater detail, in a preferred embodiment of the invention, a housing, which is used in the formation of an optical package, includes a back wall for receiving a photodiode and corresponding RF circuitry, which components may be mounted, for example, as a single xe2x80x9cmonolithic millimeter integrated circuitxe2x80x9d (MMIC) chip secured to the wall. A housing front wall, which may be generally parallel to and spaced from the back wall, includes a first aperture positioned to be generally opposite to the photodetector. The aperture is closed by a first closure, or lid, and an optical fiber assembly is adjustably secured to the lid. A top housing wall, extending between and hermetically sealed to the front and back walls, includes a second, or viewing, aperture for providing access to the interior of the housing and a second lid, or closure, is provided to seal this aperture. Side and bottom walls hermetically sealed to the front, back and top walls complete the housing enclosure.
To fabricate the photonics package of the present embodiment, the MMIC chip is mounted in the housing, and the first closure is secured to the front wall of the housing. This first closure includes a fiber alignment window which is generally aligned with the photosensitive surface onto which optical signals are to be directed. An end portion of a jacketed optical fiber which is to supply optical signals to the photodetector preferably is hermetically sealed in a coaxial, elongated ferrule, with the free, or distal, end of the fiber extending beyond the ferrule. The ferrule is then inserted into a coaxial flange to form an optical fiber assembly. This assembly is positioned in the fiber alignment window, and the distal end of the fiber is accurately aligned with the photodetector surface for maximum coupling efficiency, preferably by the use of an active automatic control system through an adjustable holding mechanism such as pneumatic tweezers.
After the optical fiber has been positioned, the ferrule surrounding the fiber is secured to the coaxial flange, as by a ring weld, hermetically sealing the joint between the flange and the ferrule. Thereafter, the flange is secured over the fiber alignment window via welding. A relatively slow-setting sealant, such as solder, can be used to provide a hermetic seal. While the sealant is setting, and is still fluid, the fiber is realigned, as needed, by the adjustable holding mechanism. As the sealant sets, the fiber is reliably and accurately aligned with the photodetector by this process, assuring maximum light signal coupling. Finally, the closure for the viewing aperture is sealed in place, to complete the improved, hermetically sealed photonics package in accordance with the present invention.